2021 No https://www.sciencedirect.com/science/article/abs/pii/S0269749121016766?via%3Dihub Zhu Asia Review Elementary (K-5) Carbon Dioxide (CO2) Carbon Monoxide (CO) Particulate Matter (PM) Volatile Organic Compounds (VOCs) Bacteria Fungi Indoor air quality Multi-center Exposure School China We observed that there were 66.5%, 52.6%, 22.4%, 1.8% and 9.6% of the classrooms that exceeded the guideline values of PM2.5, PM10, CO2, HCHO, and bacteria, respectively.